FIG. 1 shows an overview of a mobile communication system 100. The system comprises several mobile terminals 110, 120 connected to several base stations 130-1, 130-2, 130-3, 130-4. The mobile terminals 110, 120 could communicate with the base stations 130 using Frequency Division Multiplex (FDD) or Time Division Multiplex (TDD). FDD means that all communication from the mobile terminal 110, 120 to the base station 130 is transmitted on one frequency band, called the uplink frequency band, and all data communication from the base station 130 to the mobile terminal 110, 120 is transmitted on another frequency band, called the downlink frequency band. TDD means that all communication between the mobile terminal 110, 120 and the base station 130 uses the same frequency band for both the uplink and downlink communication. In this case the uplink and downlink communication is separated in time. If FDD is used the communication could be either Half Duplex or Full Duplex. Half Duplex means that the mobile terminal 110, 120 is not transmitting and receiving at the same time. In Full Duplex the mobile terminal 110, 120 and the base station 130 can receive and transmit at the same time.
The base stations 130 are connected to Radio Network Controllers (RNC) 140-1, 140-2, and the RNCs 140 are connected to a Core Network 150. In this application the RNC is defined as a node in the network that controls one or several base stations. This functionality can be included is the base stations or be a separate node in the network.
A scheduler for controlling the communication between the mobile terminal 110, 120 and the base station 130 is placed in the base station 130 or in the RNC 140, not shown in the figure. The scheduler determines where in the time- and frequency domains the mobile terminal 110, 120 and the base station 130 should transmit and receive its resources. Resources are in this context defined as the data or control information to be sent. Usually the scheduler does not allocate the whole frequency band to one user. The frequency band is divided in smaller parts called frequency carriers. In Global System for Mobile communication (GSM) one frequency carrier is 200 kHz and in Universal Mobile Telecommunication System (UMTS) one frequency carrier is 5 MHz. In Long Term Evolution (LTE) the smallest system bandwidth is 1.4 MHz and control signaling is spread over the entire system bandwidth (BW), but the smallest frequency carrier that can be allocated to one user is 180 kHz. In LTE this is called a resource block and consists of 12 sub carriers on 15 kHz BW each. The scheduler allocates both uplink frequency carriers and downlink frequency carriers. The distance between an uplink carrier and a downlink carrier allocated to one user is called the duplex distance
FIG. 2 shows a part of a transceiver 200 according to prior art. The transceiver can be located in a mobile terminal 110, 120 or in a base station 130. When the mobile terminal 110, 120 or the base station 130 is transmitting the signal is transformed to Radio Frequency (RF) signal in the RF Application Specific Integrated Circuit (RF ASIC) 210. The signal is transformed to the frequency of the uplink frequency carrier that shall be transmitted over the air to the base station 130. Even if a majority of the signal energy is within the frequency carrier bandwidth, a small amount of energy will leak to the neighbor frequencies. To minimize this leakage, the leakage power is filtered by for example a Surface Acoustic Wave (SAW) filter 220. The power of the signal is amplified in the Power Amplifier (PA) 230 before being transmitted through the antenna 250 via the duplexer 240. The duplexer isolates the receiver (RX) from the transmitter (TX) in FDD-mode and consists basically of two SAW-filters. Normal TX to RX attenuation is 40-50 dB.
When the mobile terminal 110, 120 or the base station 130 is receiving, the signal is received at antenna 250 and forwarded to the Low Noise Amplifier (LNA) 260 via the duplexer 240.
Even if the uplink frequency carrier and the downlink frequency carriers are spaced apart by the duplex distance some energy will leak from the transmitter (TX) to the receiver (RX) and increase the noise in the receiver. A majority of this noise can be removed by filters such as SAW filter 220 but some energy will still leak to the receiver through the duplexer 240. The leakage from the transmitter to the receiver will increase with high transmit power.
Also, distortion caused by external interferers, the transmitted signal, and the receiver nonlinearities may position unwanted tones in the receive band and thus degrade the receiver signal-to-noise-ratio (SNR).
The problem with the existing solution is associated with cost. The external inter-stage SAW filter 220 is expensive, and concepts to avoid it may be power-hungry and difficult to implement. If the SAW filter is removed this will increase the linearity requirements on the transmitters especially at high power levels.
Another problem is that the number of external SAWs will increase with increased band-support, thus giving a big cost penalty for multiple-band transceivers.